The present invention generally relates to an electrophotographic copying machine of a type having a plurality of magnification modes and, more particularly, to an exposure adjustment for controlling the amount of the exposure falling on a photo-receptor surface in the electrophotographic copying machine to suit to a particular magnification mode.
Three types of exposure adjustment means for an electrophotographic copying machine of a type having a plurality of magnification modes have heretofore been well known. The first type is so designed as to adjust the width of an exposure slit or the intensity of light emitted by an illumination device, for example, an electric lamp, for adjusting the density of an image to be reproduced according to the density of an original image. The second type is, for example, disclosed in any one of the U.S. Pat. No. 3,438,704, patented on Apr. 15, 1969; the U.S. Pat. No. 3,883,244, patented on May 13, 1975; the U.S. Pat. No. 3,917,393, patented on Nov. 4, 1975; and the Japanese Laid-open Patent Publication No. 52-147424, laid open to public inspection on Dec. 7, 1977. This second type is generally so designed as to adjust the width of an exposure slit according to change of the magnification, thereby avoiding the possibility of variation in the amount of the exposure falling on the photo-receptor surface through the exposure slit which may occur as a result of the change of the magnification.
The third type is, for example, disclosed in the U.S. Pat. No. 4,125,323, patented on Nov. 14, 1978, (corresponding to the Japanese Laid-open Patent Publication No. 52-146630, laid open to public inspection on Dec. 6, 1977) and is so designed as to give the exposure slit a larger width at the center thereof than at the opposite ends thereof according to a change of the magnification, thereby to avoid variation in the amount of the exposure falling on the photo-receptor surface through the exposure slit, which may take place in a direction longitudinally of the exposure slit upon change of the magnification under the influence of the aperture efficiency of a projecting lens and the cosine law of the optical characteristic of the projecting lens.
The present invention pertains to an improvement in the third type of exposure adjustment referred to above. For the purpose of the present invention, the conventional exposure adjustment of the third type referred to above will be discussed with particular reference to FIGS. 1 and 2.
In FIG. 1, there is schematically illustrated an electrophotographic copying machine comprising a transparent support 1 for the support of an original 2 to be reproduced thereon, an illuminating lamp 3 supported for reciprocal movement between a stand-still position and a scanned position and adapted to be energized during the movement thereof from the stand-still position to the scanned position, a first reflective mirror 4 supported for movement together with the illuminating lamp 3 at a predetermined velocity V, and a second reflective mirror 5 supported for movement in a direction parallel to the direction of movement of the first reflective mirror 4 at a velocity which is equal to half the velocity V of movement of the first reflective mirror 4, that is, V/2. An image of the original 2 which has been scanned by the first reflective mirror 4 is projected onto a photoconductive surface of a photo-receptor drum 9, driven in one direction at a velocity equal to the velocity V of movement of the first reflective mirror 4, by way of third and fourth reflective mirrors 7 and 8 after having passed through a projecting lens 6. The intensity of the illumination carrying the original image and falling on the photoconductive surface of the rotating photo-receptor drum 9 is adjusted in its longitudinal direction by an exposure slit 12 defined by a pair of elongated slit defining members 10 and 11, prior to reaching the photoconductive surface of the photo-receptor drum 9.
The foregoing description applies where the copying machine is in a full-size reproduction mode, that is, in position to reproduce an image having a size equal to the size of the original image. However, a reduced size reproduction mode to reproduce an image having a size equal to, for example, 0.707 times the size of the original image can be attained by repositioning the projecting lens 6, the third reflective mirror 7 and the fourth reflective mirror 8 from the respective positions shown by the solid lines to positions shown by the broken lines and, at the same time, by causing the first and second reflective mirrors 4 and 5 to be driven at respective velocities of V/m and V/2m, wherein m represents the magnification, that is, 0.707 in this example. In addition, an additional slit defining member 13 having a shape as shown in FIG. 2 has to be pivoted to an operative position so as to protrude into the path of travel of rays of light carrying the original image to be projected onto the photoconductive surface of the photo-receptor drum 9. As best shown in FIG. 2, the additional slit defining member 13 is of a type having a width which is smaller at a central portion than at either of the opposite ends thereof so that, when the slit defining member 13 is in the operative position, it cooperates with the slit defining member 11 to define an exposure slit 12 having a shape having a width which is larger at the central portion than that at either of the opposite ends of said slit 12. With the additional slit defining member 13 in the operative position, variation in intensity of the illumination occurring in a longitudinal direction of a exposure slit by which the intensity of the illumination falling on the surface of the photo-receptor drum 9 through either of the opposite end portions of the exposure slit 12 is higher than that of the central portion of the same slit can advantageously be avoided and also an increase of intensity of illumination within the entire slit.
The reason the provision of the additional slit defining member 13 is necessary will now be described. In general, in an electrophotographic copying machine having the construction described above, it is well known that the intensity E' of the illumination falling on the photoconductive surface of the photo-receptor drum 9 and the amount L of exposure to which the photoconductive surface of the photo-receptor drum 9 is actually exposed can be expressed by the following formulae. ##EQU1## wherein Eo represents the intensity of illumination used to illuminate the original 2 placed on the transparent support 1; .beta. represents the magnification (e.g., 0.707); .eta. represents the aperture efficiency of the projecting lens 6; .theta. represents the half angle of view; F represents the aperture ratio of the projecting lens 6; and C.sub.1 represents a constant which is determined by the following formula: EQU C.sub.1 =K.multidot..tau..multidot.n.multidot.m
wherein K represents the reflection factor of the original, .tau. represents the transmission factor which is the sum of the transmission factors of the projecting lens and the transparent support, n represents the number of reflective mirrors employed and m represents the reflection factor of the reflective mirrors. The product of the aperture ratio F multiplied by the sum of one plus the magnification, that is, F.multidot.(1+.beta.), represents the effective F-number which is generally determined by the F-number of the projecting lens 6 and the magnification .beta.. It is also to be noted that the product of the aperture efficiency .eta. of the projecting lens 6 multiplied by cos.sup.4 .theta. represents the illuminating ratio of the projecting lens which is determined by the aperture efficiency .eta. of the projecting lens and the fourth power of the cosine of the half angle of view .theta.. EQU L=E'.multidot.(1/Vo) (2)
wherein d represents the width of the slit 12 and Vo represents the peripheral velocity of the photo-receptor drum 9.
On the other hand, the relationship between the illuminating ratio of the photoconductive surface of the photo-receptor drum and the distance from the optical axis is illustrated in FIG. 3 and it is clear from FIG. 3 that the illuminating ratio of the photoconductive surface of the photo-receptor drum decreases with an increase of the distance from the optical axis. Accordingly, it is obvious that, in order for the photoconductive surface of the photo-receptor drum to be uniformly exposed, the intensity of illumination directed onto the original supported by the transparent support should increase in such a manner as shown by the broken line in FIG. 3 with an increase of the distance from the optical axis. It is to be noted that the description concerning FIG. 3 is applicable where the projecting lens 6 employed has a focal length f of 280 mm.
Where the original is illuminated in the manner as discussed with reference to FIG. 3, the photoconductive surface of the photo-receptor drum will be uniformly exposed. This is particularly true where the copying machine is in the full-size production mode. However, when the copying machine is subsequently changed to the reduced size reproduction mode, not only does the effective F-number of the projecting lens vary to such an extent that the intensity of the illumination which may fall on the photoconductive surface of the photo-receptor drum may vary, but also the half angle of view varies incident to the repositioning of the projecting lens, which may result in variation in intensity of the illumination at local areas of the photoconductive surface of the photo-receptor drum.
By way of example, in the construction shown in FIG. 1, assuming that the width of the exposure slit 12 is fixed, the ratio of the amount of exposure to which the photoconductive surface of the photo-receptor drum is exposed during the reduced size reproduction mode relative to the amount of exposure to which the photoconductive surface of the photo-receptor drum is exposed during the full-size reproduction mode can be calculated as follows. ##EQU2## In other words, during the reduced size reproduction mode, the amount of exposure to which the photoconductive surface of the photo-receptor drum is exposed is higher by 37% than that during the full-size reproduction mode, tending to give rise to over-exposure.
In addition, the ratio of the amount Lo of exposure to which the photoconductive surface of the photo-receptor drum is exposed through a central portion of the exposure slit relative to the amount L.sub.150 of exposure to which the photoconductive surface of the photo-receptor drum is exposed through either of the opposite end portions of the exposure slit, both occurring during the reduced size (x0.707) reproduction mode, can be calculated as follows when the original is illuminated by light of an intensity equal to that during the full-size reproduction mode. ##EQU3## In other words, the amount of exposure falling on either of the end portions of the photo-receptor drum is higher by about 9% than that falling on a central portion of the photo-receptor drum, tending to give rise to an irregular concentration of the reproduced image. It is to be noted that in calculating the ratio of the amount Lo of exposure relative to the amount L.sub.150 of exposure, no variation in intensity of the illuminating light emitted from the lamp is taken into consideration and, if this variation in intensity of the illuminating light emitted from the lamp, which often occurs in practice, is taken into consideration, the difference in intensity of the illumination between the central portion and either of the opposite end portions of the photo-receptor drum will be larger than that calculated in the above manner.
In view of the above, in the electrophotographic copying machine of the type disclosed in the U.S. Pat. No. 4,125,323, the invention of which has been assigned to the same assignee of the present invention, the slit defining member of the type described above and shown in FIG. 2 is necessarily employed. However, so far as two magnification modes, that is the full size reproduction mode and the reduced size reproduction mode, are concerned, the exposure adjustment by means of the additional slit defining member 13 in addition to the slit defining members 10 and 11 such as disclosed in the U.S. Pat. No. 4,125,323, is satisfactory. However, where the number of magnification modes is three or more, a correspondingly increased number of slit defining members are required and this in turn requires a complicated mechanism for selectively bringing the slit defining members into an operative position one at a time.